Conventionally, a communication apparatus equipped with an array antenna is often used in a mobile radio communication system. This communication apparatus equipped with an array antenna is an apparatus with a plurality of antennas spatially distributed for controlling directivities of received signals based on information received by those antennas. A conventional communication apparatus equipped with an array antenna will be explained below by taking a case where this communication apparatus is mounted on a base station apparatus as an example with reference to FIG. 1.
FIG. 1 illustrates a configuration of a base station apparatus equipped with a communication apparatus incorporating a conventional array antenna and an appearance of a radio communication system including this base station apparatus and mobile station apparatus. FIG. 1 shows a case where the above-described base station apparatus has two antennas.
A radio communication between base station apparatus 10 and mobile station apparatus 20 is carried out according to the following procedure. First, a signal sent by mobile station apparatus 20 is received by base station apparatus 10 via reception antenna 11 and reception antenna 12.
The signals received by reception antenna 11 and reception antenna 12 at base station apparatus 10 are each subjected to predetermined processing such as frequency conversion by RF section 13 and RF section 14. The signals subjected to the above-described processing by RF section 13 and RF section 14 are each subjected to A/D conversion processing and despreading processing by A/D conversion/despreading section 15 and A/D conversion/despreading section 16 and then output to array combining/signal processing section 17.
The input signals from A/D conversion/despreading section 15 and A/D conversion/despreading section 16 are each subjected to array processing by array combining/signal processing section 17, then subjected to signal processing such as RAKE combining or error correction and reception data is obtained. Furthermore, array combining/signal processing section 17 multiplies the above-described input signals by complex coefficients during the above-described signal processing to control directivities during reception. Through this, base station apparatus 10 obtains directivities, which are strong in the directions of desired signals and weak in the directions of interference signals, thus making it possible to obtain reception data with influences of interference signals reduced. That is, array combining/signal processing section 17 can obtain reception data free of influences of interference.
However, the characteristics of RF section 13 and RF section 14 vary depending on characteristic differences of analog elements incorporated and fluctuate every moment due to variations with time and temperature, etc. The same applies to A/D conversion/despreading section 15 and A/D conversion/despreading section 16.
Thus, the reception data obtained when the above-described components are used as they are comprises the signals received by reception antenna 11 and reception antenna 12 subjected to mutually different unknown amplitude fluctuations and phase rotations. As a result, base station apparatus 10 may have difficulty in acquiring originally intended directivities.
Therefore, in order to prevent the above-described phenomenon, processing is carried out which consists of measuring characteristics of RF section 13, RF section 14, A/D conversion/despreading section 15 and A/D conversion/despreading section 16 and determining complex amplitudes to be multiplied by array combining/signal processing section 17 taking into account the respective characteristic differences. Such processing is called “calibration”.
Calibration is carried out as follows. First, calibration radio signal generator 30 sends a calibration radio signal. This calibration signal is received by base station apparatus 10 via reception antenna 11 and reception antenna 12.
The signals received by the above-described antennas at base station apparatus 10 are subjected to processing similar to that described above by RF section 13 and A/D conversion/dispreading section 15, and RF section 14 and A/D conversion/dispreading section 16.
Calibration processing section 18 measures characteristic errors using signals output from the respective A/D conversion/despreading sections. Furthermore, during a normal communication, such correction values that allow array combining/signal processing section 17 to cancel out characteristic errors that exist in RF section 13 and A/D conversion/dispreading section 15, and RF section 14 and A/D conversion/dispreading section 16 are stored in a correction table in calibration processing section 18. This is the method for carrying out calibration.
After calibration, array combining/signal processing section 17 carries out the above-described signal processing to minimize influences of characteristic errors that exist in RF section 13, RF section 14, A/D conversion/dispreading section 15 and A/D conversion/dispreading section 16 with reference to the correction table in calibration processing section 18.
The following two types of method are available as the methods for combining normal communication and calibration. The first method is the one that provides a time to execute calibration (process a radio signal for calibration) in the communication system as a whole in addition to the time for a normal communication. That is, during a time provided besides the time for a normal communication, calibration radio signal generator 30 sends a calibration radio signal and base station apparatus 10 carries out calibration using this calibration radio signal.
The second method is the one that executes calibration simultaneously with a normal communication. That is, calibration radio signal generator 30 always sends a radio signal for calibration and base station apparatus 10 performs calibration using this calibration radio signal while carrying out a communication with mobile station apparatus 20.
However, the communication apparatus equipped with a conventional array antenna has the following problems. First, in the case where the above-described first method is used, base station apparatus 10 must interrupt a communication with mobile station apparatus 2 to carry out calibration. This deteriorates the processing efficiency in a communication between base station apparatus 10 and mobile station apparatus 2.
Further, in the case where the above-described second method is used, since calibration radio signal generator 30 always sends a calibration radio signal, the signal that base station apparatus 10 receives from mobile station apparatus 20, which is a communication terminal apparatus, always contains interference components from this calibration radio signal.
That is, since the signal sent from mobile station apparatus 20 to base station apparatus 10 always receives interference from the above-described calibration radio signal, the signal received by base station apparatus 10 is already affected by interference for a long period of time. For this reason, the received signal at base station apparatus 10 contains errors continuously for a long period of time, which is likely to make communications between base station apparatus 10 and mobile station apparatus 20 difficult.